Actually this program allows you design trusses. Trusses are composed
of straight members connected at their ends by hinged connections to form
a stable configuration. When loads are applied to a truss only at the joints,
forces are transmitted only in the direction of each of its members. That
is, the members experience tension or compression forces, but not bending
forces. Trusses have a high strength to weight ratio and consequently are
used in many structures, from bridges, to roof supports, to space stations.

In this simulation,
trusses are created by attaching members to nodes
(joints). First, nodal locations are specified; then the nodes are linked
by members to create a structure. Once the structure is established, two
of the nodes must be assigned as support nodes. One must be a "fixed"
node, i.e., one that can provide support in both the x- and y-directions;
the other must be a "rolling" node, one that can provide support
in only the y-direction. Finally, one or more nodes can be assigned to
bear loads.

Once these elements are specified,
a click on "Calculate" will check your design. Another click
will generate a complete force diagram showing compression/tension forces
in each of the members and reactive forces at the support nodes.

This simulation works only for statically determinate, stable trusses.
A necessary (but not sufficient) criterion for a determinate, stable structure
is M + 3 = 2*N , where M is the number of members and N is the number of
nodes. The simulation will not calculate forces unless this condition is
met.

Nodes, members, support nodes, and loads can be added or removed at
any time. After each change of configuration, a click on "Calculate"
will recalculate the forces. If elements are missing, or the condition
for a determinate, stable structure is not met, a diagnostic message will
describe the problem. (Note: for display purposes, loads should not exceed
100.)